Adaptation of Ralstonia solanacearum biovar 2 to temperate climates

PhD ceremony: Ms. P. Stevens, 13.15 uur, Academiegebouw, Broerstraat 5, Groningen

Thesis: Adaptation of Ralstonia solanacearum biovar 2 to temperate climates

Promotor(s): prof. J.D. van Elsas

Faculty: Mathematics and Natural Sciences

Ralstonia solanacearum biovar 2, the causative agent of potato brown rot, probably originates from the cradle of potato, the Andes and from there has incidentally spread to different regions of the world. In the Netherlands, bacterial wilt has been reported as from the mid-1990-ies. The organism can still be found in local waterways. Patricia Stevens hypothesized that the organism, which is of tropical and warm temperate origin, may have experienced conditions that are strongly selective for particular genotypic and/or phenotypic adaptations.

To investigate whether the Dutch R. solanacearum population has diversified, strains were isolated by Stevens from water, bittersweet plants and sediment from a high risk area. These newly obtained strains (n=42) were genetically and phenotypically analyzed and compared to two strains isolated from potato, i.e one Dutch and one tropical strain. Overall, the strains showed a clonal structure using a range of molecular methods. However, heterogeneity across strains was seen that appeared mostly related to recombination processes. Stevens identified a 17.6 Kb putative genomic island (PGI-1), which was deleted from the genome of environmental strain KZR-5. Although the exact set of functions harbored by PGI-1 is as yet unknown, the island encompasses two genes of potential ecological relevance, i.e. one encoding a hypothetical protein with a RelA/SpoT domain and a putative cellobiohydrolase. Stevens thus assessed the behavior of several strains under different environmental conditions. Her results indicate that environmental strain KZR-5 has incurred altered fitness, with divergent outcomes, i.e. enhanced tolerance to cold stress versus reduced competitiveness during growth and in plant invasion.